Which quantity represents the energy required to change a substance from solid to liquid at its melting point?

• A. Latent heat of vaporization Lv
• B. Latent heat of fusion Lf
• C. Specific heat capacity Cp
• D. Temperature change ΔT

Answer: (B)

The energy required to change a solid into a liquid at its melting point is called the latent heat of fusion. At the melting point, added heat goes into breaking the forces holding the solid together, allowing particles to flow past one another and form a liquid, rather than raising the temperature. The amount of energy needed per unit mass is Lf, and the total energy for a sample is Q = mLf.

This differs from latent heat of vaporization, which is used for turning a liquid into a gas, and from the idea of specific heat capacity, which measures energy needed to raise temperature without a phase change. Temperature change ΔT itself is not energy.